Photochromic nitrobenzylpyridines



United States Patent 6 3,290,281 PHOTOCHROMIC NITROBENZYLPYRIDINES Julius Weinstein, Framingham, Aaron L. Bluhm, Canton, and John A. Sousa, Framingham, Mass, assiguors to the United States of America as represented by the Secretary of the Army N Drawing. Filed July 18, 19.63, Ser. No. 296,136 9 Claims. (Cl. 260-443) The invention described herein, if patented, may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to a novel group of photochrornic derivatives of Z-nitrobenzylpyridine.

The novel compounds of this invention have the following general formulae:

fear H 1 wherein X represents a radical selected from the group consisting of hydroxyl, nitrile, thiocyanate, amido, lower alkyl substituted amido, aryl substituted amido, amino, lower alkyl substituted amino, aryl substituted amino, earboxylic acid, lower valkyl ester of carboxylic acid, aryl ester of carboxylic acid, halogen, nitro, thiol, lower alkyl sulfide, aryl sulfide, sulf-onic acid, lower alkyl ester of sulfonic acid, aryl ester of sulfonic acid, alkoxy, phenoxy, lower alkyl, aryl, alkoyl and aroyl radicals and Y represents a radical selected from the group consisting of hydroxyl, nitrile, thiocyanate, amido, lower alkyl substituted amido, aryl substituted amido, amino, lower alkyl substituted amino, aryl substituted amino, carboxylic acid, lower alkyl ester of carboxylic acid, aryl ester of carboxylic acid, halogen, nitro, thiol, lower alkyl sulfide, aryl sulfide, sulfonic acid, lower alkyl ester of sulfonic acid, aryl ester of sulfonic acid, alkoxy, phenoxy, lower alkyl, aryl, alkoyl, aroyl, lower :alkyl azoxy, aryl azoxy, lower alkyl azo and aryl azo; and wherein Z represents a radical selected from the group consisting of hydroxyl, nitrile, thiocyanate, amido, lower alkyl substituted amido, aryl substituted a-rnido, carboxylic acid, lower alkyl ester of carboxylic acid, aryl ester of carboxylic acid, halogen, thiol, lower alkyl sulfide, aryl sulfide, sulfonic acid, lower alkyl ester of sulfonic acid, aryl ester of sulfonic acid, alkoxy, phenoxy lower alkyl, aryl, alkoyl, aroyl, lower alkyl azoxy, aryl azoxy, lower alkyl azo and arylazo.

The compounds described above possess in common the ability to change or develop color in a solution phase when exposed to radiation in the ultra violet and visible light range. This color change is reversible, i.e., upon withdrawal of the radiation source or when placed in darkness the compounds return to their unactivated state. This property, known as reversible photochromism, can be employed to great advantage in light filtering devices. For example, compounds which exhibit this property may be incorporated in transparent plastic sheets which can be used alone or laminated to glass as wind-ow materials which will automatically develop a color and darken upon exposure to bright sunlight and will become colorless and clear in dim light. This built-in automatic light screen or filter in windows eliminates the need for other types of light screens heretofore employed for this purpose. By incorporating these compounds in the windows of satellites Patented Dec. 6, 1966 ICC it is possible to achieve some measure of internal temperature control by screening out some of the infra-red ray-s. An important military application for these compounds is the protection of individuals from the unexpected blinding flash of nuclear weapons. Unprotected observers of a nuclear explosion are temporarily or permanently blinded by the intense ultra violet and visible light radiation accompanying the explosion. A transparent plastic, e.g., polymethylmethacrylate, eye shield containing the compounds of this invention would darken with suificient rapidity in the presence of a nuclear flash to minimize or prevent any serious eye injury from the visible and near visible radiation.

Within the above class of compounds certain are preferred because of their ability to screen or absorb radiation over a wider spectrum and in particular to screen the higher energy visible and near visible light waves, or because of the rapidity with which they fade from their activated state when the light source is withdrawn, or because of their ability to be activated while in the solid crystalline state, or because of ability to resist decomposition when exposed to prolonged irradiation. These preferred compounds are 2-(2-nitro-4 cyanobenzyl) pyridine, Z-(Z-nitro- 4-carbomethoxy-benzyl) pyridine, 2-(2-nitro 4 carboethoxybenzyl) pyridine, 2-(2-nitro-4-amidobenzyl) pyridine, 2-(2-nitro-carboxybenzyl) pyridine, 2-(2-4dinitro-ahydroxybenzyl) pyridine, 3,3-dinitro-4,4'-di (PyridylmethyD-azoxybenzene, and 4-(2-nitro-4 cyanobenzyl) pyridine.

The preparation of the preferred compounds is described in the following examples. Preparation of other novel compounds of this invention can be accomplished by making the required substitutions which are well within the skill of the art.

EXAMPLE I 2-(2'-nitr0-4'-cyan0benzyl) pyridine 3.0 g. (0.013 mole) of pure 2-(2'-nitro-4'-aminobenzyl) pyridine, prepared according to the procedure of Nunn and Schofield, J. Chem. Soc., 583 (1952), were added to 3.4 ml. (0.039 mole) of concentrated hydrochloric acid in 5.0 ml. of water and the stirred solution was cooled to 0 C. The amine was diazotized by the gradual addition of 0.90 g. (0.013 mole) of sodium nitrite in a small volume of water. The diazonium solution was added to a magnetically stirred mixture of 1.17 g. (0.013 mole) of cuprous cyanide, 1.70 g. (0.026 mole) of potassium cyanide, 10 ml. of water and 100 ml. of toluene at 5l0. The reaction mixture was allowed to Warm gradually to room temperature, heated for 1 /2 hours at C., cooled and made basic with 5% sodium hydroxide. Solids were collected on a filter. They and the water layer of the filtrate were extracted several times with toluene. The combined extracts were washed with 3% sodium hydroxide, water, and then dried over anhydrous magnesium sulfate. Evaporation of the toluene gave 1.5 g. of brown residue which quickly crystallized. Repeated recrystalli zation from ethanol using charcoal and from isooctanebenzene gave colorless crystals, M.P. 102l03. On exposure to light the compound became green in color and in the absence of light the compound returned to the colorless state. The infrared spectrum showed the presence of a C-EN group in the molecule.

Analysis.--Calcd. for C H O N C, 65.26; H, 3.79; N, 17.56. Found: C, 65.16; H, 3.86; N, 17.62.

EXAMPLE II 2-(2-nitro-4-carb0meth0xybenzyl pyridine Dry hydrogen chloride gas was passed through 3 ml. commercial absolute methanol in a 10 ml. round bottom flask fitted with a condenser protected with a calcium chloride drying tube and a gas inlet tube, for 10 min. To

q u this solution 0.20 g. of the cyano compound, prepared in Example I, was added and the solution heated over a steam bath for 3 hr. On cooling, a solid (the hydrochloride) deposited. To this mixture 3 ml. water was added and the solution made alkaline with 28% ammonium hydroxide. The crystals which deposited were filtered after chilling and washed with water. Recrystallization from aqueous methanol furnished colorless lancelets melting at 67-68 in 75% yield. The infrared spectrum showed absence of CEN group, and the presence of a carboxylate group. On exposure to light the crystalline form hecame dark blue-green in color which color quickly faded in the dark.

Analysis.-Calcd. for C H N O C, 61.76; H, 4.44; N, 10.29. Found: C, 61.49; H, 4.32; N, 10.33.

EXAMPLE III 2-(2-nilr0-4-carb0ethoxybenzyl) pyridine Dry hydrogen chloride gas was passed through 3 ml. commercial absolute ethanol in a ml. round bottom flask fitted with a condenser protected with a calcium chloride drying tube and a gas inlet tube, for 10 min. To this solution 0.20 g. of the cyano compound, of Example I, was added and the solution heated over a steam bath for 3 hr. On cooling, a solid (the hydrochloride) deposited. To this mixture 3 ml. water was added and the solution made alkaline with 28% ammonium hydroxide. The crystals which deposited were filtered after chilling and washed with water. Recrystallization of the product from aqueous ethanol furnished white crystals melting at 72-73 in 83% yield. These crystals temporarily turned a pale blue-green upon exposure to light.

Analysis.Calcd. for C H N O C, 62.93; H, 4.93; N, 9.79. Found: C, 62.63; H, 4.70; N, 9.65. 1

EXAMPLE IV 2-(2-nitr0-4-amid0benzyl) pyridine In a 50 ml. Erlenmeyer flask 0.30 g. of the cy-ano compound, prepared in Example I, was dissolved in 15 ml. acetone and 2 ml. 10% hydrogen peroxide Was added. The solution was placed in the dark at room temperature for 2 days and then evaporated in 'a current of air. Pale yellow crystals deposited which were filtered and washed with water. Recrystallization from 50% aqueous ethanol furnished cream colored needles melting at 169170 .5 in 70% yield. The infrared spectrum showed absence of CEN group, and the presence of a carbonyl group at 1685 cmf The NH group was observed at 3120 and 3305 cm? in a Halocarbon mull and at 3411 and 3528 cm.- in chloroform solution. Irradiation of crystals caused the development of a pale green color.

Analysis.--Oalcd. for C H O N C, 60.69; H, 4.31; N, 16.33. Found: C, 60.40; H, 4.43; N, 16.40.

EXAMPLE V 2-(2-nitr0-4-carb0xybenzyl) pyridine 2-(2-nitro-4-carbomethoxybenzyl) pyridine (0.25 g.) was refluxed with 18% hydrogen chloride acid for 8 hr. The cooled solution deposited fine white needles (hydrochloride) which were filtered and washed with a little ice water. This product was dissolved in 15 ml. warm water and just neutralized with 5% sodium hydroxide. The white solid which precipitated was filtered and washed with a small amount of ice water and recrystallized from 95% ethanol, M.P. 208-210", white, 0.10 g. The irradiated solid form of this compound is pale gray in color.

Analysis.-Calcd. for C H N O C, 60.46; H, 3.90; N, 10.85. Found: C, 60.26; H, 3.91; N, 10.98.

EXAMPLE VI 2-(2,4-dinitr0-u-hydr0xybenzyl) pyridine In a 500 ml. round bottom flask fitted with a condenser, stirrer and addition funnel, a suspension of 2.75 g. (0.010 mole) of 2-(2,4-dinitrobenzoyl) pyridinein 150 ml. commercial absolute methanol was cooled to 0 and an ice cold solution of 1.04 g. (0.028 mole) sodium borohydride in 15 ml. methanol was quickly added. The reddish-purple mixture was stirred at 0 for 1 hr. and then the temperature allowed to rise. -After. 1.5 hr. at room temperature the reaction mixture was heated at 50 for 1 hr. Dilute sulfuric acid was added and the inorganic material filtered. The filtrate was evaporated to one-fifth its original volume, an equal volume of water added and the solution basified with 2% sodium hydroxide. This mixture was extracted with ether, the ether extracts dried over anhydrous magnesium sulfate, and the drying agent removed. Evaporation of the ether afforded a yellow solid. Recrystallization from ethanol furnished pale yellow crystals melting at 134.55 (dec.). The infrared spectrum showed absence of (:0 band and the presence of OH band.

Analysis.Calcd. for C H N O C, 52.37; H, 3.30; N, 15.27. Found: C, 52.62; H, 3.36; N, 15.13.

EXAMPLE VII 3,3'-dinitr0-4,4-di (Z-pyrzdylmethyl) -az0xybenzene In a ml. Erlenmeyer flask 5 g. of 2-(2-nitro-4-aminobenzyl) pyridine (M.P. 118.5, prepared according to the procedure of Nunn and Schofield), 40 ml. glacial acetic acid, 10 ml. 30% hydrogen peroxide and 2 drops concentrated sulfuric acid were combined. The mixture was kept at room temperature for 48 hours, and 50 ml. water was added followed by basification with 20% sodium hydroxide. An oil which separated was extracted with toluene, the toluene extract washed with water and dried over anhydrous magnesium. sulfate. After removal of the drying agent, the toluene was removed in a current of warm air. The solid yellow residue was recrystallized with a charcoal treatment from aqueous ethanol. The pure compound was yellow and melted at 143-1435 (70% yield). The infrared spectrum showed no NH absorption bands. The crystalline form develops a bluegreen color upon irradiation with visible light.

Analysis.Calcd. for G T-1 N 0 C, 61.27; H, 3.86; N, 17.87. Found: C, 61.36; H, 3.97; N, 18.01. Molecular weight. Calcd.: 470.43. Found: 464.0 (vapor pressure method).

EXAMPLE VIII 4-(2-nitro4-cyan obenzyl) pyridine A magnetically stirred solution of 1.5 g. (0.0065 mole) of 4-(2-nitro-4-aminobenzyl)-pyridine (M.P. 129130, lit. 1) and 1.68 ml. (0.020 mole) of concentrated hydrochloric acid in 5 ml. water was cooled to 0 and the amine diazotized by the gradual addition of 0.45 g. (0.0065 mole) of sodium nitrite in 4 ml. of Water. The diazonium solution was then added to a magnetically stirred mixture of 0.58 g. (0.0065 mole) of cuprous cyanide, 0.85 g. (0.013 mole) of potassium cyanide, 5 ml. of water, and 75 m1. toluene at 5-10. The reaction mixture was heated at 80 for 2.5 hr., cooled, and made basic with 5% sodium hydroxide. Solids were collected on a filter. They and the water layer of the filtrate were extracted several times with toluene. The combined extracts were washed with 3% sodium hydroxide, water, and then dried over anhydrous magnesium sulfate. Evaporation of the toluene afforded 0.63 g. amber residue which rapidly crystallized. Several crystallizations from tolueneligroin (charcoal treatment) furnished pale yellow crystals melting at 7374. The infrared spectrum showed the presence of CEN group.

Analysis.Calcd. for C H N O C, 65.27; H, 3.79. Found: C, 64.89; H, 3.95.

As previously indicated, all of the described compounds undergo a color change rapidly when exposed to visible or ultra violet light. For example, any of the preferred compounds in solution phase show immediate color development when exposed to a light pulse of 0.5 microsecond duration. High optical densities of the color are easily obtainable. For example, optical densities of 2.0 were observed when dilute solutions (10" moles/ liter) of 2-(2- nitro-4-cyanobenzyl) pyridine in ethanol contained in a ZO-cm. cell were irradiated with a light pulse having an 5 energy of 2,000 joules and a duration of 5.0 microseconds. According to Beers Law relationships, the same optical density would be obtained utilizing a cell with a path length of 1.0 mm. and a solution with a concentration of 2 l0 moles/liter. Higher optical densities would of course be obtained with more concentrated solutions.

Having thus described our invention we Wish it to be understood that the description and specific examples are to be taken as illustrative only and not for purpose of limiting the scope of our invention, inasmuch as a number of variations and modifications will readily occur to the expert with-out departing from the spirit of our invention.

We thus intend to claim our invention broadly and to that end define the same by the appended claims.

We claim:

1. A photochromic compound of the (following formula wherein X is a member selected from the group consisting of hydrogen and hydroxy and Y is a member selected from the group consisting of cyano, carbo-lower alkoxy, carhoxa mido, carboxy,

seeoqpunpuno References Cited by th e Examine;

UNITED STATES PATENTS 10/1951 Sperber et a1. 260-296 4/1962 Walter 260-296 FOREIGN PATENTS 11/ 1953 Great Britain.

OTHER REFERENCES Margerum et al.: J. Phys. Chem, volume 66, pp. 2434- 2438 (1962).

CHARLES B. PARKER, Primary Examiner.

JOSEPH P. BRUST, Examiner.

F. D. HIGEL, Assistant Examiner. 

1. A PHOTOCHROMIC COMPOUND OF THE FOLLOWING FORMULA 